Investigation on the influencing factors of synergistic microwave pyrolysis of oil sludge by metal oxide clay minerals

ABSTRACT

This investigation focused on the application of microwave pyrolysis technology, utilizing metal oxide clay minerals as both catalysts and microwave absorbers, with the goal of enhancing resource utilization efficiency in the treatment of oily sludge. Experimental results demonstrated that incorporating metal oxide clay minerals into the microwave pyrolysis process at 2000 W power substantially improved the yield of pyrolysis oil and gas, while simultaneously optimizing the products’ chemical composition. This study elucidates, for the first time, the distinct catalytic roles of three metal oxide clay minerals—hematite (Fe₂O₃), apatite (P₂O₅), and corundum (Al₂O₃)—in the microwave pyrolysis process. The findings indicate that these catalysts effectively increase the proportion of light fractions in pyrolysis oil and significantly reduce the production of harmful gases.The composition of combustible gases (H₂ + CH₄ + CO) in the pyrolysis gases exhibited increases of 7.224 wt%, 2.831 wt%, and 18.895 wt% for hematite, apatite, and corundum, respectively, relative to the control group. Specifically, hematite markedly enhanced the gas yield, particularly the concentrations of H₂ and CO, and improved the quality of the pyrolysis oil, thereby augmenting the sludge’s energy recovery value. Furthermore, apatite exhibited outstanding nitrogen removal efficacy during the pyrolysis process, contributing to a reduction in nitrogen content in the pyrolysis oil and consequently diminishing the associated environmental risks.

Keywords:
Oily sludge; Microwave pyrolysis; Absorbing medium; Metal oxide clay mineral

1. INTRODUCTION

As China’s economy continues to expand rapidly, the demand for petroleum resources has exhibited a consistently upward trajectory. Often referred to as the “lifeblood” of modern industrial societies, petroleum is crucial in fostering economic growth and satisfying energy requirements. However, the processes of extraction, transportation, storage, and refining of petroleum resources invariably generate substantial quantities of solid organic waste, commonly known as oily sludge [¹]. Oily sludge, a complex amalgam predominantly consisting of oil, water, solid particles, and various organic compounds [²], presents significant disposal challenges. This substance not only consumes extensive land areas but also poses severe risks to the ecological environment and human health due to the ease with which the oil and organic compounds it contains can contaminate soil and water bodies [³]. Concurrently, oily sludge is laden with organic hydrocarbon components [⁴], from which substantial quantities of crude oil can be extracted, underscoring its significant potential for resource recovery. Consequently, developing effective methods to manage oily sludge and mitigate its environmental impacts represents a critical and urgent challenge.

Recent studies have elucidated a variety of treatment strategies for oil sludge, notably solvent extraction, biological treatment, incineration, and surfactant methods [⁵]. Although each treatment technology possesses unique advantages, they also exhibit inherent limitations; for instance, the extraction technology requires significant financial investment and frequently leads to secondary pollution when treating large-scale oil sludge. The incineration method incurs substantial investment costs and has a high propensity to result in resource wastage. The biological method, characterized by prolonged operational periods, proves unsuitable for treating high oil content or undertaking large-scale sludge treatment projects. Tempering treatment technology, still in its developmental phase, shows poor adaptability, is prone to causing secondary pollution, and poses substantial risks to groundwater resources. Chemical surfactants, recognized for their toxicity, pose environmental hazards, whereas biosurfactants, despite their environmental benefits, are impeded by high costs and limited availability. It is imperative that we adopt measures to ameliorate these negative impacts. Relative to the aforementioned treatment technologies, pyrolysis is more frequently employed, representing one of the most extensively researched and industrially applied technologies for the resource utilization of oily sludge. Pyrolysis efficiently heats sludge to temperatures ranging from 400–1000°C within an inert atmosphere, inducing thermal decomposition of organic materials to generate gaseous and liquid hydrocarbons. These products facilitate the rational utilization of resources [⁶], with the selective extraction of specific outputs achievable through the precise regulation of pyrolysis conditions. FÉLIX et al. [⁷] reported that pyrolysis oil can be fractionated into diesel, gasoline, and fuel oil, offering promising alternatives to low-grade petroleum resources. The resultant pyrolysis gas, abundant in hydrogen and carbon monoxide, is particularly well-suited for syngas production and serves as a foundational feedstock in the chemical industry. Moreover, the solid residue holds significant potential as an adsorbent, soil conditioner, or as an additive in construction materials. These versatile applications not only enhance resource utilization efficiency but also substantially amplify the practical relevance and industrial applicability of this research. SILVA et al. [⁸] reported that biochar derived from biomass through pyrolysis not only holds significant promise in agricultural applications but also serves effectively as a soil conditioner and pollutant adsorbent, thereby enhancing resource efficiency and contributing to environmental sustainability. Pyrolysis treatment serves as a potent method for resource recovery, adept at extracting high-value organic components from oily sludge, and demonstrates broad applicability across various sludge types. Pyrolysis yields oil-phase components whose properties closely mimic those of petroleum [⁹], facilitating easy storage and transport, and offering potential as an alternative to low-grade petroleum resources. This technique generates minimal nitrogen and sulfur compounds during processing, concurrently immobilizing heavy metals within the solid-phase byproducts, thereby mitigating environmental risks [¹⁰]. However, the effectiveness of pyrolysis may vary across different types of oily sludge, and the process is notably energy-intensive, rendering it economically challenging for large-scale deployment [¹¹].

Microwaves represent high-frequency electromagnetic waves, distinct from traditional heating methodologies. This technology facilitates uniform material heating via dielectric losses in an electromagnetic field, with energy propagation occurring through space or media as electromagnetic waves [¹²]. Microwaves selectively cleave macromolecules, including colloids and bitumen in oil sludge, diminishes toxic polymers, and enhance the oil’s condition and properties, thereby advancing the objectives of utilizing, minimizing, and detoxifying oil sludge resources [¹³]. However, the microwave-induced cracking of oil sludge tends to be selective for the heating medium within the microwave electromagnetic field environment. Given the heating characteristics of dielectric materials, microwaves effectively heat polar substances. Their efficacy diminishes for nonpolar substances with specific molecular structures, even in the absence of direct heating. Experimental findings reveal that the inclusion of microwave absorbers—such as SiC, Fe₂O₃, graphite, activated carbon, and pyrolysis coke powder—into the heating materials significantly enhances the cracking effect [¹⁴,¹⁵,¹⁶]. SALEMA et al. [¹⁷] discovered that incorporating biochar, which possesses favorable dielectric properties, into oil sludge markedly accelerates the microwave cracking’s heating rate and promotes hydrocarbon decomposition. HO et al. [¹⁸] investigated the impacts of conventional electric and microwave heating on the roasting outcomes of ground coffee and microalgae residue. Their findings indicate that microwave heating, by virtue of its rapid heating rate and time efficiency, significantly outperforms conventional electric heating, thereby enhancing the productivity of the roasting process. MOTASEMI and GERBER [¹⁹] conducted a simulation study revealing that at a higher microwave power setting (e.g., 600 W), biomass pellets within the reaction zone exhibit the fastest heating rates, attain higher maximum temperatures, and achieve optimal energy utilization efficiency, thus demonstrating microwave power’s substantial efficacy in heating and energy utilization. GOMEZ et al. [²⁰] employed microwave pyrolysis to process refinery sludge, achieving a reduction of up to 5% in weight within merely five minutes, concurrently enhancing the material’s handling characteristics and its overall composition.

In the selection of catalysts for microwave pyrolysis, metal oxide clay minerals are preferred due to their superior microwave absorption characteristics. Additionally, their unique crystal structure endows these minerals with a high specific surface area, offering numerous active sites that facilitate catalytic reactions. Metal oxide clay minerals exhibit enhanced microwave absorption properties, which promote the effective transfer and utilization of microwave energy, thereby augmenting the efficiency of microwave pyrolysis. Furthermore, the distinctive crystal structure and extensive specific surface area of metal oxide clay minerals furnish copious active sites for pyrolysis reactions, enhancing both the yield and quality of pyrolysis oil, while diminishing the production of deleterious by-products. SONG et al. [²¹] explored the catalytic pyrolysis of coal volatiles using two types of hematite minerals and observed that these minerals considerably decreased tar yield and increased the production of methane, carbon dioxide, and hydrogen, thus optimizing the gas product composition. Additionally, the iron atoms and lattice oxygen in hematite play pivotal roles in the cracking and deoxygenation reactions of tars. Specifically, iron atoms facilitate the cleavage of C–C and C–H bonds in hydrocarbons, while lattice oxygen aids in breaking the C–O bonds, thereby fostering the production of carbon dioxide. ZHANG et al. [²²] explored the catalytic effects of hematite in pyrolysis, noting significant increases in the production of gases like CO₂, CO, and H₂. This catalyst also enhanced ring-opening reactions and the cleavage of macromolecular intermediates, leading to elevated yields of alcohols, phenols, esters, and ethers. Furthermore, hematite substantially enhances deoxygenation during the polycondensation stage, aiding in the transformation of oxygenated compounds into benzene derivatives and other small molecules, and decreasing the apparent activation energy required for pyrolysis, thus augmenting both efficiency and product quality. GRUSELLE et al. [²³] conducted research on apatite-based catalysts, revealing their efficacy in catalyzing a diverse array of chemical reactions, such as oxidation, reduction, bond formation (e.g., C–C, C–O, C–S, C–N), cycloaddition, and multicomponent reactions. XIANG et al. [²⁴] discovered that apatite catalysts possess notable thermal stability, a mesoporous structure, and a high specific surface area, coupled with robust ion-exchange capabilities and tunable surface acid-base properties. The plentiful hydroxyl groups on its surface facilitate modification by polar functional groups, making it suitable for diverse reactions including hydrogenation, oxidation, condensation, and transesterification, with exceptional efficacy in both liquid and gas-phase systems. VLASKIN et al. [²⁵] observed that using corundum tubes for the pyrolysis of methane at 1100°C resulted in a hydrogen yield of 73.35%, significantly surpassing that achieved with other materials. SU et al. [²⁶] indicated that corundum exhibits outstanding chemical stability under both reducing and oxidizing conditions, making it particularly well-suited for high-temperature methane pyrolysis processes. Additionally, increasing the surface roughness of corundum enables the creation of more active sites, which efficiently encourage carbon deposition and notably enhance hydrogen production. This reaction pathway presents considerable promise for resource recovery and the repurposing of organic waste generated during petroleum extraction and processing, given its considerable compositional parallels to biomass. Given their ability to efficiently cleave C–C, C–H, and C–O bonds, metal oxide clay minerals not only enhance the yields of light tars and gases but also, due to their excellent thermal stability, extensive specific surface area, and robust ion exchange capabilities, facilitate diverse reactions. This enhances catalyst selectivity and reactivity, increases hydrogen production, and mitigates carbon accumulation issues. Consequently, metal oxide clay minerals effectively facilitate the breaking and reorganization of C–C, C–H, and C–O bonds, enabling dehydrogenation and hydrogenation reactions on the catalyst’s surface to produce olefins and saturated hydrocarbons. Simultaneously, free radicals and small hydrocarbons undergo condensation and polymerization reactions, forming larger hydrocarbon molecules. This underscores the utility of metal oxide clay minerals in enhancing yields and generating larger molecules during microwave pyrolysis of sludge. This process also results in increased production of combustible gases, which can be subsequently harnessed as energy sources to enhance overall energy utilization efficiency. Accordingly, this study aims to explore the wave-absorbing and catalytic properties of various metal oxide clay minerals as microwave absorbers and catalysts. It further seeks to assess their impact on improving the quality and conversion rates of pyrolysis products during microwave pyrolysis of oil sludge, utilizing GC-MS for analysis.

2. MATERIALS AND METHODS

2.1. Materials

The characteristics of the waste oil sludge raw material were determined, and the elemental and physical compositions were ascertained through tests conducted according to the methods outlined in Table 1.

Table 1
Composition analysis of oily sludge.

ELEMENT COMPOSITION (WT%)					PHYSICAL COMPOSITION
C	H	O	N	S	Moisture content (wt%)	Oil length (wt%)	Calorific value (MJ/Kg)	Ash content (wt%)
36.42	15.79	23.09	8.3	1.16	38.5	23.03	33.57	17.24

The oil sludge used in this study was sourced from the landed oil sludge of the Daqing Oilfield in Heilongjiang Province, China. Figure 1 illustrates the static phenology of the oil sludge, indicating that this landed oil sludge exhibits low viscosity and high fluidity. It resembles a mixture of soil and sand and is characterized by a black-brown color.

Figure 1
Real image of oil sludge.

2.2. Oil sludge microwave pyrolysis system design

Figure 2 presents a schematic diagram of the microwave pyrolysis system for light oil and combustible gas recovery, comprising a microwave oven, an integrated quartz reactor, a high-precision thermocouple, condensation, a gas-collecting bag, and an N₂ bottle. The experimental microwave oven, supplied by Henan Cole Microwave Innovation Technology Co, Ltd., features a power range from 0 to 2200 watts and a frequency of 2450 ± 50 Hz. It is equipped with a 30 mm aperture at the center and left side of the end cap to accommodate a U-shaped glass tube. This tube is inserted through these apertures into the furnace cavity and is connected to an internal quartz reactor, facilitating internal heating and external atmospheric condensation. The condensed pyrolysis oil is captured in a PET transparent bottle, while the pyrolysis gas is collected in a gas bag produced by Dalian Delin Company [²⁷]. To ensure experimental safety, a microwave detector is positioned outside the experimental microwave oven to continuously monitor microwave radiation in real time. The microwave leakage was maintained below 5 mW/cm², complying with international safety standards. Prior to the experiment, a nitrogen bottle was attached to the left side to purge the air from the quartz reactor, thereby ensuring an inert atmosphere. A high-precision thermocouple was affixed to the outer wall of the quartz reactor to facilitate real-time temperature measurements of the materials within during the pyrolysis process. The microwave oven features a PLC system that automatically adjusts the temperature and power settings, with the real-time measured temperature prominently displayed on the PLC’s screen.

Figure 2
Schematic diagram of experimental equipment for microwave pyrolysis.

2.3. Experimental process

Prior to initiating the experiment, Nitrogen (N₂) was introduced into the reaction chamber at a flow rate of 500 mL/min to ensure that the experiments were conducted in an oxygen-free environment. During the experiment, N₂ was continuously introduced at a fixed flow rate and the pyrolyzed volatiles were discharged into a condensing unit. Each experimental batch comprised 200 g of oil sludge, precisely weighed using an electronic balance and placed into the quartz reactor within the microwave oven for uniform heating. A catalyst amounting to 10% of the oil sludge mass was added to enhance pyrolysis [²⁸]. To ensure thorough mixing of the oil-containing sludge and catalyst, the sludge was first preheated to 50°C to decrease its viscosity, followed by the addition of the measured oil sludge to the crucible. The samples were homogenized using a mechanical stirrer, with the stirring speed maintained at 1000 rpm for 15 minutes. Subsequently, the furnace door was closed, the microwave power was set to 2000 watts, and an air collection bag was attached at the outlet of the loading end. The microwave oven was then activated to commence the experiment. Temperature readings were systematically recorded every minute from the start of the experiment until the completion of the pyrolysis process. During the microwave pyrolysis process, the high-temperature oil and gas were condensed via the condensation tube, with the pyrolysis oil collected in a condensation bottle. Non-condensable gases were processed through a gas washing device and subsequently collected. The gas collection bag was replaced every three minutes until the reaction concluded. Following completion, the microwave oven door was opened to allow heat dissipation, and the contents of the reaction vessel were cooled to room temperature before being weighed. To ensure the reliability of the experimental data, each experiment was conducted three times, and the resulting data were averaged for further analysis.

2.4. Pyrolysis products and analytical methods

The products resulting from the microwave pyrolysis of oil sludge are categorized into three distinct fractions: pyrolysis oil, pyrolysis gas, and solid residue carbon. The primary constituents of the pyrolysis gas include hydrogen (H₂), carbon monoxide (CO), methane (CH₄), carbon dioxide (CO₂), and C₂ to C₄ short-chain alkanes and olefins. Pyrolysis oil primarily comprises hydrocarbons, alcohols, lipids, and aromatic compounds. This oil can be fractionated into diesel, gasoline, and fuel oil through distillation processes.

To detect pyrolytic oil, an appropriate sample amount was first taken and dissolved in dichloromethane, followed by a 20-fold dilution. Ultrasonic extraction was then performed using a dedicated machine for testing. The analysis was carried out employing the highly reliable Agilent gas chromatography-mass spectrometry (GC-MS) with the advanced Agilent 19091S-433 model. The GC-MS system was equipped with an HP-5 fused silicon capillary column, measuring 30m × 250μm × 0.25μm, ensuring precise separation of components. A sample size of 1μL was utilized, and helium was used as the carrier gas at a flow rate of 1.0 mL/min, ensuring smooth and efficient chromatographic performance. The initial temperature of the gas chromatography was set at 50°C, which was gradually increased to 100°C after 3 minutes at a rate of 10°C/min. Subsequently, it was further raised to 280°C at a rate of 15°C/min for 4 minutes and finally reached 320°C at a rate of 30°C/min. The mass spectrometer settings included electron shock mode with an electron energy of 70ev, filament current of 34μA, doubling voltage of 2117 V, and a complete scan. The identification of compounds was accomplished by meticulously comparing the data with the comprehensive NIST mass spectrometry library, ensuring accurate and reliable results. Moreover, the relative contents of the samples were determined using area normalization, providing a quantitative assessment of each component’s concentration.

3. RESULT AND DISCUSSION

3.1. Microwave catalyzed pyrolysis mechanism

The microwave pyrolysis process is primarily divided into two key phases: the thermal cracking and the catalytic pyrolysis reaction sections. During microwave pyrolysis, the oil-laden sludge initially undergoes a thermal cracking reaction as the internal temperature increases. Chain hydrocarbons within the sludge are internally dehydrogenated and cracked, leading to the cleavage of C–C and C–H bonds and subsequent formation of olefins, with the concurrent release of methane (CH₄), hydrogen (H₂), and other light hydrocarbon gases. Concurrently, oxygen-containing compounds within the sludge undergo deoxygenation reactions to yield olefins, emitting carbon monoxide (CO) and carbon dioxide (CO₂). Subsequently, upon reaching high temperatures (approximately 500°C) in the microwave, the olefins undergo further dehydrogenation and polymerization reactions to form aromatic hydrocarbons, a process referred to as thermal cracking. Aromatic hydrocarbons produced through thermal cracking, along with those initially present in the oil-containing sludge, are adsorbed onto the surface of clay minerals. A portion of these hydrocarbons is then degraded via steam reforming to generate clean gases, including hydrogen (H₂). The remaining aromatic hydrocarbons continue to undergo cracking to form light hydrocarbons. Concurrently, certain polycyclic aromatic hydrocarbons are adsorbed onto the surface of silicate clay minerals and undergo dehydrogenation polymerization. This process contributes to catalyst deactivation and defines the reactive phase of catalytic pyrolysis.

3.2. Effect of metal oxide clay minerals on the warming process

Figure 3 displays the real-time temperature variation curves for the pyrolysis of oily sludge following the addition of three types of catalysts—corundum, apatite, and hematite—at a microwave power setting of 2000 watts. Analysis of the figure reveals that the temperature rise trends for the oil-containing sludge in the control group (without catalysts) and the samples with added catalysts are similar, with temperatures increasing over time in both cases. Further analysis of the figure indicates that the addition of various catalysts enhances the pyrolysis efficiency of oily sludge. Notably, hematite reduces the pyrolysis time the most significantly and increases the final pyrolysis temperature from 600°C in the control group to 800°C. The heating trend depicted in the figure can be categorized into five distinct stages within the heating curve of the oil sludge. (1) Rapid Heating: The temperature swiftly escalates to 100°C within the first 10 minutes. (2) Dehydration Stage: Between 100°C and 150°C, the oil-containing sludge undergoes dehydration, marked by significant water vapor volatilization upon reaching 100°C. (3) Hydrocarbon Volatilization Stage: From 150°C to 500°C, this stage is characterized by the volatilization of hydrocarbons within the sludge. (4) Pyrolysis Stage: The temperature range from 500°C to 600°C marks the pyrolysis stage of the oil-containing sludge. (5) Carbonization Stage: Between 600°C and 800°C, the oil-containing sludge enters the carbonization stage.

Figure 3
Temperature rise curve of microwave pyrolysis sludge of metal oxide clay minerals.

The rapid warming observed in the initial stage is primarily due to the volatilization of water from the oily sludge and its effective absorption of microwave energy. The high dielectric constant of water induces dielectric polarization and internal molecular friction, culminating in the rapid generation and release of heat energy, rapidly reaching 100°C. Subsequently, the condenser’s temperature rises to 100°C. At this temperature, water droplets are observed continuously condensing and dripping from the condenser tube, with substantial water vapor evaporation noted, as depicted in Figure 4 Once the water has fully evaporated, the temperature steadily increases to 150°C, initiating the volatilization of hydrocarbons within the sludge. With further temperature increases, the light hydrocarbon components within the oil sludge are progressively released in significant quantities. As the temperature reaches between 280°C and 370°C, a substantial volume of flue gas begins to be emitted from the end of the condenser tube. Upon gas release, the oil sludge transitions into the thermal cracking stage, where heavy oil components start to decompose, resulting in the continuous production of light hydrocarbons and small molecular compounds. As the temperature escalates to 600°C, the reactions intensify markedly, producing voluminous thick smoke at the end of the condenser tube. The color of the gas, potentially containing hazardous gases such as H₂S and NO, indicates the severity of the reactions. As the temperature continues to rise, reaching 650°C, the reaction stabilizes and the smoke begins to disperse. The culmination of sludge pyrolysis is the carbonization stage, where the temperature plateaus at approximately 750°C, ceasing to increase further. As illustrated in Figure 4, the choice of catalyst markedly influences both the rate of temperature increase and the ultimate temperature reached during pyrolysis. Notably, the final temperatures attained during pyrolysis varied among the different catalysts. The final temperatures of pyrolysis, ranked from highest to lowest, were as follows: hematite (Fe₂O₃), apatite (P₂O₅), corundum (Al₂O₃), and the blank group. Hematite (Fe₂O₃) exhibited the highest pyrolysis rate, primarily due to its notable dielectric and magnetic loss properties, which promote effective energy absorption and conversion in the microwave field, thereby accelerating reaction kinetics [²⁹]. Additionally, the strong interactions between Fe₃+ ions and Fe-O bonds create extensive charge transfer pathways at active sites, further facilitating the pyrolysis process. The optimized crystal structure and refined particle morphology of hematite also enhance its microwave absorption efficiency, leading to a marked increase in pyrolysis performance [³⁰]. High-temperature pyrolysis likely induces numerous crystal structure defects in iron oxide catalysts, enhancing their adsorption capacity and catalytic activity, which in turn improves overall performance. Other metal oxide clay minerals also significantly reduced pyrolysis time and increased the pyrolysis rate compared to the control group.

Figure 4
Real-time temperature phenomenon diagram during microwave pyrolysis of oil sludge.

3.3. Influence of metal oxide clay mineral type on the yield of pthermolysis products

Figure 5 illustrates that the presence of metal oxide clay minerals markedly influenced the distribution of pyrolysis products from oil-containing sludge when subjected to microwave heating at 2000W. This observation underscores the critical role of the microwave absorber type in determining the yield distribution of products post-pyrolysis. In comparison to the blank group devoid of corundum, the gas yield from oil-containing sludge experienced a substantial increase following the incorporation of corundum (Al₂O₃) into the microwave pyrolysis process. At the peak pyrolysis temperature, the gas yield reached 42.1%, representing an 18.7 percentage point increase over the blank group. A direct correlation exists between the increased gas yield and the decreased solid residue yield, demonstrating a clear positive relationship. The addition of hematite (Fe₂O₃) resulted in the highest pyrolysis oil yield of 30.5%, which exceeded that of the blank group by 9.4%.The gas production followed the order: corundum (Al₂O₃) > hematite (Fe₂O₃) > apatite (P₂O₅) > blank group. The oil production, ranked in descending order, was hematite (Fe₂O₃) > apatite (P₂O₅, CaO) > corundum (Al₂O₃) > blank group.

Figure 5
Yield distribution of oily sludge after microwave pyrolysis under different catalysts.

In summary, the use of metal oxide clay minerals as microwave absorbers consistently enhanced the yield of pyrolysis oil and gas in all tested groups, except for the corundum (Al₂O₃) group, which showed no significant improvement in pyrolysis oil yield. However, the corundum (Al₂O₃) group demonstrated a substantial increase in gas production. This enhancement is attributed to corundum (Al₂O₃) facilitating further evaporation, secondary cracking, or thermal decomposition of the oil. Metal oxide clay minerals can absorb more microwave energy compared to other absorbers, efficiently converting it into thermal energy, thereby increasing the pyrolysis temperature and heating rate. The swift pyrolysis of oily sludge further contributes to the generation of oil fractions.

It was observed that the use of metal oxide clay minerals as microwave absorbers resulted in a notable increase in gas production in the corundum (Al₂O₃) group during pyrolysis, despite the lack of a substantial increase in pyrolysis oil yield. This phenomenon is attributed to the catalytic effect of corundum (Al₂O₃), which promotes further evaporation, cracking, or thermal decomposition of oil molecules. Compared to other microwave absorbers, metal oxide clay minerals excel in absorbing microwave energy and converting it into thermal energy. This process directly enhances the pyrolysis temperature and rate, thereby facilitating the rapid generation of oil fractions.

3.4. Effect of metal oxide clay mineral type on pyrolysis gas composition

Figure 6 illustrates the catalytic effect of metal oxide clay minerals on various gas components during microwave pyrolysis of oily sludge at a heating power of 2000W. The figure reveals that CO₂ and H₂ constitute the primary pyrolysis gas components with high concentrations, whereas the concentrations of light hydrocarbons (CxHy) are relatively low. The addition of metal oxide clay minerals markedly increased the total content of combustible gases (H₂, CH₄, CO). This result further corroborates the pivotal catalytic role of metal oxide clay minerals in the microwave catalytic cracking process of oily sludge.

Figure 6
Effect of adding different silicate clay minerals on pyrolysis gas composition.

3.4.1. Catalytic cracking of oily sludge on apatite

The incorporation of apatite into the microwave pyrolysis sludge markedly enhanced the content of combustible gases, notably increasing the concentrations of H₂ and CO while reducing the concentrations of CO₂ and CH₄. Figure 7 presents the XRD characterization of apatite prior to pyrolysis, revealing that apatite exhibits a singular phase of Ca₅(PO₄)₃, with no detectable characteristic peaks of other oxides. The characteristic peaks of Ca₅(PO₄)₃ are identified at 2θ = 25.78°, 31.98°, 32.14°, 34.05°, and 39.98°. The composition includes 55.38% CaO and 42.06% P₂O₅. During the microwave heating of apatite, phosphate ions (PO₄)₃ lose some oxygen atoms at high temperatures, leading to a reaction between phosphate and oxygen as depicted in Equation (4–1), with calcium oxide (CaO) as the primary product. Under elevated temperature conditions, phosphate ions (PO₄)₃ undergo further decomposition, as illustrated in Equation (4–2), resulting in the formation of simpler phosphorus oxide compounds, with phosphorus pentoxide (P₂O₅) being the predominant product. Hence, calcium oxide (CaO) and phosphorus pentoxide (P₂O₅) exert a predominant influence on the pyrolysis of oil sludge. The incorporation of CaO facilitates the cracking of light hydrocarbons in oil sludge [³¹], resulting in the increased production of H₂, as illustrated in Equation (4–3). Additionally, CaO reacts with water vapor to yield hydrogen (H₂) and calcium hydroxide (Ca(OH)₂). This phenomenon occurs because calcium oxide can react with water vapor at elevated temperatures, forming hydrogen and calcium hydroxide, as depicted in Equation (4–4). Phosphorus pentoxide (P₂O₅) possesses the capability to adsorb carbon dioxide (CO₂) and facilitate its reduction to carbon monoxide (CO), as illustrated in Equation (4–5). Moreover, the phosphorus element in calcium phosphate may participate in the cracking and gasification reactions of tar, leading to the formation of carbon monoxide and hydrogen. Concurrently, CaO, as a basic oxide, can react with CO₂ to form CaCO₃, thereby absorbing the CO₂ generated during microwave pyrolysis of oil sludge, and consequently reducing the CO₂ yield. The reduced CH₄ yield may be attributed to the CaO-mediated gas-phase conversion of CH₄ to CH₃OH, as depicted in Equations (4–6).

Figure 7
XRD spectra of apatite before pyrolysis.

(equation 4.1) $${\text{Ca}}_5{({\text{PO}}_4)}_3\to 5\text{CaO}+{\text{P}}_4{\text{O}}_6+{\text{O}}_2$$

(equation 4.2) $${\text{P}}_4{\text{O}}_6\to 2{\text{P}}_2{\text{O}}_5$$

(equation 4.3) $${\text{C}}_{\text{n}}{\text{H}}_{\text{x}}\to \text{C}+{\text{C}}_{\text{m}}{\text{H}}_{\text{y}}+{\text{H}}_2$$

(equation 4.4) $$\text{CaO}+{\text{H}}_2\text{O}\to \text{Ca}{(\text{OH})}_2+{\text{H}}_2$$

(equation 4.5) $${\text{P}}_2{\text{O}}_5+{\text{CO}}_2\to 2\text{CO}+{\text{O}}_2$$

(equation 4.6) $$\text{CaO}+\text{CH}4\to \text{Ca}+{\text{CH}}_3\text{OH}$$

3.4.2. Catalytic cracking of oily sludge in corundum

As demonstrated in Figure 5, the hydrogen (H₂) content increased by 8.42% following the addition of corundum compared to the control group, indicating that corundum serves a pivotal role as a catalyst in the microwave pyrolysis of oil sludge. This pronounced increase underscores the diverse catalytic effects of corundum. Pre-pyrolysis corundum was characterized using X-ray diffraction (XRD), with the scanning angle set at 2θ: 10°–80° after data processing as illustrated in Figure 8 It is evident from these results that γ-Al₂O₃ was detected in the corundum, as further corroborated by Figure 9 γ-Al₂O₃ is classified as a porous medium, characterized by a rich pore structure and extensive surface area, providing numerous active reaction sites, thereby enhancing the catalytic efficiency of microwave pyrolysis. Active reaction sites, thereby facilitating the catalytic reaction. The porous structure also enhances the contact area between the catalyst and reactants, thereby improving reaction efficiency. Additionally, corundum exhibits high stability, resistance to high temperatures and corrosion, and its optimal porosity can create channels for microwave-incidence materials, facilitating the enhancement of impedance matching between the absorber and electromagnetic waves. Impedance matching is critical for microwave absorption performance, enabling rapid elevation to higher temperatures. Given that the C–C bond breaks easily while the C–H bond remains intact at lower temperatures, the incorporation of γ-Al₂O₃ facilitates microwave absorption, enabling rapid elevation of the reaction temperature. This acceleration of C–H bond cleavage promotes dehydrogenation, thereby increasing H₂ content and releasing more abundant hydrogen. Furthermore, oil-containing sludge possesses a high water content, and water molecules can actively participate in the reaction during the cracking process. Consequently, the catalytic cracking of oil-containing sludge on corundum (γ-Al₂O₃) can be considered a hydrocarbon water-gas reaction and a steam reforming reaction [³²], as illustrated in Equations (4–7) and (4–8). Corundum exhibits a high CO₂ adsorption capacity, effectively sequestering the generated CO₂ and consequently lowering its concentration in the gas phase. This reduction in CO₂ content addresses the issue of greenhouse gas emissions post-pyrolysis of oil-containing sludge.

Figure 8
XRD spectra of corundum before pyrolysis.

Figure 9
Corundum SEM scanning electron microscope.

(equation 4.7) $${\text{C}}_{\text{x}}{\text{H}}_{\text{y}}+{\text{XH}}_2\text{O}\to \text{XCO}+\left(\frac{x+y}{2}\right){\text{H}}_2$$

(equation 4.8) $$\text{CO}+{\text{H}}_2\text{O}\to {\text{CO}}_2+{\text{H}}_2$$

3.4.3. Catalytic cracking of oily mud stains on hematite ore

The microwave pyrolysis of oil sludge with hematite addition significantly increased the gas content, notably elevating the H₂ and CO components while reducing the CO₂ and CH₄ components compared to the control group. Hematite facilitated the cracking of light hydrocarbon organic matter and promoted CO₂ reforming (4–9) and steam reforming reactions (4–8). Additionally, hematite effectively absorbs microwave energy, enhancing the Joule heating effect due to its high thermal conductivity, thereby improving temperature uniformity and the efficiency of microwave heating reactions. Furthermore, hematite may possess specific electromagnetic properties that enhance its microwave absorption capacity. Under microwave heating conditions, hematite effectively absorbs microwave energy and converts it into thermal energy, thereby promoting the pyrolysis reaction of the sludge and increasing the yield of combustible gas. The XRD characterization of hematite prior to pyrolysis, depicted in Figure 10, reveals that hematite exhibits solely a single physical phase of Fe₂O₃ without the characteristic peaks of other oxides. The characteristic peaks of hematite were detected at 2θ = 24.28°, 33.15°, 35.78°, 40.68°, and 49.24°. The results demonstrated that hematite exhibits significant catalytic activity in microwave-catalyzed pyrolysis, particularly for CO₂ reforming and water vapor reforming reactions, resulting in a decrease in CH4 content and an increase in CO content. During the catalytic process, Fe₂O₃ is reduced to elemental Fe by reducing gases such as H₂ and CO [³³]. This elemental Fe then serves as the primary catalytic active site for tar cracking, playing a crucial role in tar decomposition. The incorporation of hematite results in higher yields of CO and H₂. Hematite possesses the ability to both adsorb CO₂ and facilitate its reduction to CO and H₂. This adsorption and reduction process enhances the yield of combustible gases, particularly when active sites are available on the catalyst surface. Syngas, primarily composed of CO and H₂, is a crucial industrial feed gas with a high calorific value, widely utilized across various industrial applications. In industrial settings, this method can be adopted to produce water gas, primarily consisting of H₂ and CO, leveraging the unique aspects of microwave heating during oil sludge pyrolysis. The introduction of hematite as a catalyst for water gas production allows for the efficient separation of CO and H₂ from the mixed pyrolysis gas. This method offers advantages such as low investment, low energy consumption, and ease of operation, leading to substantial economic and social benefits. Compared to traditional methods of water gas production, this approach not only meets environmental protection requirements but also reduces costs and process complexities. This is of great practical significance for the efficient and clean conversion and utilization of waste oil sludge.

Figure 10
XRD spectra of hematite before pyrolysis.

(equation 4.9) $${\text{CH}}_4+{\text{CO}}_2\to 2\text{CO}+2{\text{H}}_2$$

3.5. Effect of metal oxide clay mineral type on pyrolysis oil composition

3.5.1. Effect of metal oxide clay minerals on pyrolysis oil composition

The component content of pyrolysis oil was analyzed using GC-MS, with the results illustrated in Figure 11. The figure demonstrates that the addition of three different metal oxide clay minerals during pyrolysis results in the main components of the oil being chain alkanes, chain olefins, aromatic hydrocarbons, and minor amounts of aliphatic compounds, cycloalkanes, and cycloolefins, which can be categorized into light and heavy components. The light oil components range from C₄ to C₁₂ and C₁₃ to C₁₈, while the heavy oil components are those greater than C₁₉. As illustrated in Figure 11, the addition of apatite and hematite results in a decrease in the heavy component content of pyrolysis oil and an increase in the light oil component content. The light component increases by 10.785% and 11.457%, respectively, compared to the control group. This increase can be attributed to the large surface area and excellent adsorption properties of nanoscale materials, which effectively reduce aromatic ring condensation and enhance the light component in pyrolysis oil. Among the tested materials, corundum increased the content of heavy oil (>C₁₉) the most compared to the control group. Corundum is a neutral catalyst carrier, which lacks sufficient acidic or alkaline sites to promote the cracking of heavy oil, unlike acidic or alkaline catalysts. Corundum, as shown in the SEM scanning electron microscope image in Figure 8, possesses a specific surface area, but it cannot promote the cracking of long-chain hydrocarbons in pyrolysis oils due to the lack of proper pore structure or active sites. However, it is noteworthy that the addition of apatite increases the light oil fraction and decreases the heavy oil fraction of pyrolysis oil. According to literature, apatite is rich in acidic sites, which promote the breaking of carbon-hydrogen bonds in hydrocarbon molecules during the cracking process, leading to an increased proportion of the light fraction. The three-dimensional pore structure of apatite can effectively adsorb and crack large hydrocarbon molecules, thereby contributing to an increased light fraction. By comparing the XRD spectra of apatite before pyrolysis in Figure 7 with the XRD spectra of hematite before pyrolysis in Figure 10, it is evident that apatite is rich in calcium (Ca), an alkaline element that provides the necessary environment for catalytic cracking. Ca+ can act as a central ion, forming a coordination field that attracts and breaks down large hydrocarbon molecules, thereby facilitating the generation of smaller hydrocarbon molecules and increasing the lightweight fraction. Apatite also contains phosphorus (P), which aids in disrupting the structure of the heavy fraction by influencing intermediates during the pyrolysis process, reducing coke production, and promoting the formation of more light molecules. The addition of hematite results in the formation of more light oils. Iron (Fe) is a highly reactive metal with excellent redox properties. During pyrolysis, iron can break carbon-hydrogen bonds through cyclic oxidation and reduction reactions, facilitating the conversion of large hydrocarbon molecules into smaller ones, thereby increasing the production of light hydrocarbons. According to the literature, hematite typically exhibits a high specific surface area and a porous structure, which facilitates the provision of numerous catalytic active sites and enhances the interaction between the catalyst and reactant molecules, thereby promoting the conversion of heavy components. In terms of cracking efficacy, the addition of metal oxide clay minerals significantly increased the yield of small molecule straight-chain hydrocarbons during the pyrolysis process, while reducing the proportion of large molecule straight-chain hydrocarbons. This indicates that these clay mineral materials effectively promote the cracking of heavy oil. Given that pyrolysis oils containing a higher proportion of straight-chain hydrocarbons are of superior quality, the use of metal oxide clay minerals, such as apatite and hematite, can enhance the quality of pyrolysis oils by increasing the proportion of lighter components.

Figure 11
Effect of the addition of different kinds of metal oxide clay minerals on the composition of pyrolysis oil.

3.5.2. Effect of metal oxide clay minerals on pyrolytic oil alcohols

After the addition of the metal oxide clay mineral catalyst, the components in the pyrolysis oil were analyzed semi-quantitatively based on the area of the chromatographic peaks obtained from GC-MS detection. The peak area content represents the relative component content in the pyrolysis oil. Following the analysis, the content of each component in the pyrolysis oil was determined. The primary alcohol components in the pyrolysis oil included 1-docosanol, nineteen-alkanol, 1-hexadecanol, and stearyl alcohol. As illustrated in Figure 12, the total content of alcohols in the pyrolysis oil increased with the addition of metal oxide clay minerals. The maximum alcohol content in the presence of apatite was 10.832%. However, the stearyl alcohol content decreased. This decrease is attributed to the phosphates in apatite, which provide a source of oxygen during the pyrolysis process. This oxygen source is crucial for the hydroxylation of hydrocarbons, a process that involves the substitution of hydrogen atoms with oxygen atoms, resulting in the formation of hydroxyl groups (−OH).The acidic catalytic environment provided by apatite facilitates this conversion by ensuring the oxidation of hydrocarbon molecules and limiting excessive cleavage, thereby making the formation of alcohols more favorable. In contrast, hematite, although effective in promoting pyrolysis reactions, primarily targets the cleavage of hydrocarbon molecules rather than oxidation reactions necessary for the formation of specific functional groups. Corundum, due to its inert nature, primarily acts as a support medium with limited capacity to directly promote chemical transformations. Thus, the phosphate component of apatite and its specific catalytic environment make it more effective in promoting alcohol generation. Stearyl alcohol, being a microtoxic substance, is slightly irritating to human skin and mucous membranes and may pollute aquatic environments. The introduction of metal oxide clay minerals as catalysts promotes the transesterification reaction of stearyl alcohols with lipids from pyrolysis oils, minimizing the content of stearyl alcohols and reducing their potential environmental and health impacts. Simultaneously, the catalytic action increases the content of other alcohols in the pyrolysis oil, thereby enhancing the overall quality of the pyrolysis oil. The increased alcohols, such as 1-hexadecanol and nineteen-alkanol, can be utilized in the production of surfactants and lubricants, as well as in optimizing the texture and color dispersion of emollient products. Thus, metal oxide clay minerals play a dual optimization role in the process: reducing harmful components and increasing the content of valuable alcohols, thereby enhancing the quality of the pyrolysis oil.

Figure 12
Effect of the addition of different kinds of metal oxide clay minerals on pyrolysis oil alcohols.

4. CONCLUSIONS

In this chapter, the microwave pyrolysis of oil-containing sludge using metal oxide clay minerals was conducted, and the pyrolysis products of the three phases were analyzed to further investigate the catalytic effect of metal oxide clay minerals on the microwave pyrolysis of oil-containing sludge. The following conclusions were drawn:

• (1)

  The addition of metal oxide clay minerals enhanced the microwave absorption capacity of the oil sludge, accelerating the heating rate and increasing the termination temperature of microwave pyrolysis. Due to its excellent wave-absorbing properties, hematite can elevate the termination temperature of pyrolysis to 760°C and reduce the pyrolysis time to 76 minutes. Additionally, the oil and gas production from pyrolysis increased with the addition of metal oxide clay minerals.

• (2)

  For pyrolysis gas, the addition of apatite and hematite increased the content of H₂ and CO components. CaO in apatite facilitated the cracking reaction of light hydrocarbons in the sludge, leading to the production of more H₂, while phosphorus pentoxide (P₂O₅) adsorbed carbon dioxide (CO2) and facilitated its reduction to carbon monoxide (CO). Hematite contributed to the cracking of light hydrocarbon organics and promoted CO₂ reforming (4–9) and steam reforming reactions. The hydrogen (H₂) content increased after the addition of corundum compared to the control group, suggesting that corundum promoted the water-gas reaction and steam reforming reaction.

• (3)

  For pyrolysis oil, the addition of metal oxide clay minerals (hematite, apatite) increased the content of light fractions (C₄-C₁₂, C₁₃-C₁₈) and decreased the content of heavy fractions (>C₁₉). This indicates that metal oxide clay minerals can effectively promote the decomposition of asphaltenes in the oil sludge, generating more low-carbon straight-chain hydrocarbons and alcohols, thereby improving the quality of the pyrolysis oil.

• (4)

  It was found that the yield of pyrolysis gas was highest with the addition of corundum, while the yield of pyrolysis oil was significantly the lowest. To increase the yield of pyrolysis oil, a catalyst based on silicate clay minerals with Al₂O₃ as the primary component of corundum was selected. This catalyst also contained various metal oxides attached to the Al₂O₃ carrier, and it was observed that the catalyst based on silicate clay minerals with the addition of corundum performed better than metal oxide clay minerals in the three-phase product analysis. The catalytic effect of the three-phase products with added silicate clay minerals was compared to that of metal oxide clay minerals.

5. ACKNOWLEDGMENTS

This work was supported by the LiaoNing Revitalization Talents Program (Grant No. XLYC2007083), the Liaoning BaiQianWan Talents Program (Grant No. LNBQW2020Q0141), the Talent Scientific Research Fund of LNPU (Grant No. 2020XJJL-010)

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Publication Dates

• Publication in this collection
  01 Nov 2024
• Date of issue
  2024

History

• Received
  19 July 2024
• Accepted
  26 Aug 2024